A high degree of specificity is proposed for the parasitic association between Lecudina and Ancora gregarines (Protozoa, Apicomplexa) and their polychaete hosts, a series of sibling species in the genus Capitella. Capitellid polychaetes are cosmopolitan indicators of highly disturbed conditions in the marine environment and ideal experimental subjects. Analysis of the gregarine parasites of capitellids will play an important role in ecological, toxicological and physiological studies of Capitella sp. It is desirable to characterize the gregarines living in the intestines of the important capitellid polychaetes, and to begin to study how they affect the population biology of their hosts. This proposal has two goals: to evaluate gregarine-host specificity by means of experimental infections, and by the electrophoretic characterization of the gregarine species; and to lay the groundwork for how gregarines may affect the population biology of the host polychaetes. We will prepare infected and uninfected cultures of the host polychaetes of interest. Experimental tests of host-parasite specificity will involve attempts to transfer infections from a given host polychaete to other species. Gel electrophoresis will be used to evaluate gregarine species distinctions thereby providing an analysis complementary to experimental infections. The problem of obtaining adequate material for analysis will be solved by comparing electrophoretic patterns of infected and uninfected host polychaetes. The ready culturability and short generation times of the critical capitellid species permit experimentation on host-parasite relationships in the group. The ease with which the Capitella species can be cultured in the laboratory, and the direct gregarine life cycles provide a model system for the first study of a series of marine host-parasite interactions in closely-related sibling host species.